1. Field of the Invention
The invention relates to curable composite materials and adhesion promoters for biomedical applications, especially dental applications.
2. Description of the Related Art
Composite materials are used for a variety of applications in the biomedical field. Objects being composed of such materials, in part or in whole, can be classified as medical devices. In the dental field, composite materials are used to restore teeth to form and function, in part, or in whole.
Dental amalgam, commonly used as a filling material for teeth, is made from a powdered alloy and mercury. The alloy primarily contains silver, tin, and copper. Other metals are present at concentrations that are relatively low compared to silver, tin or copper. The alloy is mixed with mercury to produce dental amalgam. The wetting of metal alloy particles by mercury produces a plastic mass that can easily be inserted into a cavity preparation and shaped, after which time it hardens. Mercury is used for dental amalgam as it is the only metal that exists in a liquid state at room temperature. It is thus capable of wetting the alloy powder, a process which initiates hardening. The presence of mercury in dental amalgam is controversial, as numerous studies have demonstrated that mercury is released during in-service use. This release has generated much interest in developing new, safe and functional alternatives to dental amalgam.
A class of materials known as composite resins are commonly used as alternatives to dental amalgam. Composite resins generally contain a polymer phase (a matrix) reinforced by inert fillers, these generally consisting of various forms of amorphous silica (SiO2), or polymorphs of silica. The fillers are bound to the matrix via silane couplers, with xcex3-methacryloxypropyl trimethoxysilane (xcex3-MPS) being the most commonly used silane. Siloxane linkages are formed to the filler on the silicon end of the coupler. The methacrylate moiety at the other end of the coupler co-polymerizes with the matrix. Modern composites additionally contain photointiators, accelerators, colorants and additives for maintaining chemical stability.
The longevity of different materials in dental restorations is important in assessing their effectiveness. Several cross-sectional studies have shown that the median age for composite restorations is 5 to 8 years, while that for amalgam restorations is 8 to 14 years. In a prospective study of up to five years duration, failure rates for composite restorations were approximately twice that of amalgam restorations. These survival relationships were similarly observed in children, where the median survival time for amalgam restorations was 5 years while that for composite restorations was 32 months. These data would clearly suggest that metal-based restorations are superior to conventional composite resins with respect to survival.
Confirmation of the superiority of metal-based direct restoratives can be additionally gleaned from two initiatives in developing alternatives to amalgam that involve the use of silver. One alternative is a pure silver powder which is cold-welded into a solid mass by hand pressure. The other alternative involves mixing a silver alloy with gallium, a metal in liquid phase at near-room temperature. The silver powder project appears to have some promise; however, the technique of placement may be time consuming, and could limit the widespread utility of the material. The gallium-containing alloy has been shown to be sensitive to water at its early stage of set, and requires coating with a varnish to avoid water contact for the first 24 hours.
The invention features a composition, which, when hardened, is suitable for application in the biomedical arts, such as in dentistry where it can be used as a filling or restorative material for teeth.
The invention also features a composition including a filler of metal, or a metal alloy, or a metal oxide, or a structure or structures having a metal coating, or combinations thereof, with or without surface modification. These surface modifications include methods to remove surface impurities, such as oxide layers or organic impurities. Surface modifications can also include the deposition of metal films on the surfaces of the fillers. The composition has excellent physical and mechanical properties on hardening.
The invention also features polymerizable compositions that contain fillers, the surface of these fillers having the ability to form bonds to the sulfur of a thiol. The thiol contains a moiety that is capable of bonding to a polymerizable component of the composition.
The invention also features new coupling agents, which form bonds between the surface of the filler structure and the polymerizable component of the composition.
The invention also features a method of preparing adhesion promoters having thiol functionalities which, on one end, form bonds with the surface of the filler structures and on the other end, are capable of reacting with a polymerizable component of the composite composition.
The invention also features a method of preparing adhesion promoters having phosphate functionalities which, on one end, are reacted with the surface of the filler structures, and on the other, are capable of reacting with a polymerizable component of the composite composition.
This invention features a composition of a composite material including a polymerizable material constituting a matrix, a filler, a coupling agent, stabilizers and an initiator system. The matrix is a binder into which particles of regular or irregular shape can be embedded, these particles being the filler. A coupling agent is a chemical structure that includes functionalities capable of reacting with different substances, the purpose of the coupling agent being to bond dissimilar materials to each other. In the composition featured, one of the functionalities is capable of polymerizing with the polymerizable matrix. Stabilizers are compounds that prevent the premature polymerization of the polymerizable material in the composition. Initiator systems include agents that initiate polymerization when subjected to an activating source. The composite can be used as a filling material in teeth, or as a material to re-establish the structure of teeth, in part or in whole, although it can be used in other applications requiring high-strength composites.
The matrix contains a material or materials having polymerizable moieties. The filler can include metal, metal alloy, or metal-oxide particles. The filler particles can be surface modified. The filler can also include any particles having surfaces that can react with the sulfur of a thiol. The filler can be dispersed in the matrix. The polymerization of the composite material can be activated by chemical methods, by heat, or by irradiation, such as a laser, or other visible or ultraviolet light source, or by microwave energy. Depending on the method of curing, the composition can be provided as a one-paste system, or it can be divided into two pastes. In the latter case, the composite hardening is chemically activated. One paste of the two-paste system contains the composite material and chemical initiator while the second paste contains the composite material and the chemical activator, which can be an amine accelerator. Single paste systems contain both an initiator and an amine accelerator in the single paste.
Surface modifications include the removal of surface impurities using various cleaning procedures, such as the treatment of the particles with acids. Surface modification also can involve the deposition of a thin silver or gold film on alloy particles. The coupling agent can be a form of a silane, but preferably, are thiol or phosphate compounds that bond metal or metal alloy particulates, or particulates consisting of oxides of metals to the polymerizable matrix. A metal oxide filler can be bonded to the matrix via a silane coupling agent.
The invention particularly features a metal, or a metal-alloy particle, or metal-oxide particle, or other ceramic particles, or any particle whose surface can form bonds with sulfur, these particles being used alone, or in combination, which are especially of interest as a filler in composites.
This invention also features a method of forming bonds between the sulfur of a thiol and the filler of the composite material. The thiol compounds have functionalities which can react with the polymerizable matrix of the composite material. The thiol can be deposited on the surface of a filler to form metal-sulfur bonds.
This invention also features a method of constructing and depositing a phosphate compound on metal, metal alloy or metal oxide particles, the phosphate compound having the capability of reacting with the polymerizable matrix of the composite material.
The compounds and methods of the invention can be used to bond particles having surfaces capable of forming bonds to the sulfur of a thiol to polymerizable components of any polymerizable material. The method includes coupling thiol compounds to a material, the thiol including a polymerizable component. Examples of this use include glass ionomer materials that have been modified to contain unsaturated groups capable of being polymerized.
Composite materials that harden when placed directly in the tooth would have improved strength using fillers such as metal, or metal alloy, or metal oxide particles. Composite materials having fillers bonded to the matrix using coupling agents containing a thiol or a phosphate moiety would have improved durability in a water environment by being more resistant to hydrolysis than silane coupled fillers.
This invention finds utility in formulating new composites containing metal, metal alloy, or metal-oxide fillers, or a combination thereof, dispersed in a matrix phase. Preferably, the surface of the filler can form a bond with the sulfur of a thiol. If the surface of the filler cannot form such a bond, then a film of a metal that can form a bond with the sulfur of a thiol can be deposited on the filler. The fillers are coupled to the composite matrix using new adhesion promoters based on alkylthiols. Coupling agents containing a phosphate functionality can be used to bond the fillers to the matrix in cases where a sulfur bond to the surface of the filler cannot be formed.
Thiols form chemical bonds to metal surfaces by the interaction of sulfur with metal (S-metal bonds). The formula for a thiol is given by:
HS-R
where R is an alkyl or substituted alkyl having from 2 to 100 carbon atoms, preferably 4 to 40 carbon atoms, and more preferably 6-20 carbon atoms. The substituent can be a terminal polar group having a specific functionality for reaction with other compounds. For example, the terminal group can be an acrylate. The number of carbon atoms and the nature of the substituent can be controlled to change the properties of the R group.
Thiols are capable of coupling metals to polymers. For example, a pendant thiol can be used to couple steel surfaces to a polymer matrix. Additionally, glycolic thioesters can form an S-metal bond, leaving chains that promote the adhesion of acrylate and epoxy adhesives. In addition to binding to the surface, a thiol group is also capable of an addition reaction across a vinyl group. Thiol-ene polymerizations such as, for example, a two component coupling-agent layer used to bond copper with epoxy and polyimide resins is more resistant to corrosion, and can provide higher adhesive strength than either component alone, even at high temperatures.
With respect to phosphates, the surface treatment of metal by phosphate is well-known as phosphating. A solution containing the free phosphoric acid, a primary metallic phosphate, and an accelerator are used in phosphating procedures. The surface physical or physico-chemical properties of metal surfaces, such as aluminum and its alloys, are modified by this procedure. Phosphate compounds can be deposited on the surface of metal or metal oxides by phosphating procedures.
Additional features and advantages of the invention will become apparent from the detailed description of the invention.